What is the formula used to calculate the new RPM for a fan?

The formula for calculating the new RPM for a fan is based on the relationship between the fan's RPM (revolutions per minute) and the airflow measured in CFM (cubic feet per minute). The fundamental principle here is that the fan speed and airflow are directly related; as the CFM changes, the RPM must adjust accordingly to maintain performance.

Using the formula old RPM multiplied by the ratio of new CFM to old CFM effectively allows you to scale the RPM in direct proportion to the change in airflow. If you're increasing the CFM, the RPM must also increase to maintain the expected airflow efficiencies and capacities of the fan. Conversely, if the CFM is decreased, the RPM needs to decrease as well. This maintains the characteristic performance curve of the fan and ensures that it operates effectively within its design parameters.

The mathematical expression mirrors the ratio of the changes, allowing for a consistent and reliable method to recalculate fan speeds when airflow needs modification. By appropriately using this formula, one can achieve the desired air movement while avoiding issues such as inefficiency or potential damage to the fan's components. This understanding is critical for HVAC professionals when designing systems or adjusting existing setups for optimal performance.